Displacement pumps are known and designed in many ways. Among the main types of displacement pumps membrane pumps, piston pumps and plunger pumps can be mentioned. A piston is defined in this text as a piston provided with one or several sealing elements, which interact with a smooth cylinder wall, whereas a plunger in this text is defined as a piston having a smooth, cylindrical outer surface, which interacts with one or several sealing elements, mounted in a cylindrical wall, which does not have to be smooth. Thus, a piston is in this context a machine element, which can function as either a piston or a plunger.
Piston pumps are probably the types of displacement pumps most frequently used. In comparison with e.g. the membrane pump the piston pump has the advantage, that it is possible to work with comparatively small surfaces exposed to the pump medium. Thus, the forces exerted on the pump housing do not have to be very large and thus, there are no high requirements as to the thickness of material in the pump housing, which consequently can be made comparatively light-weight. On the other hand it is necessary to work with comparatively large stroke lengths. Along all this length the pump housing cylinder must be very smooth in order to be able to interact efficiently with the sealing device or sealing devices on the pump piston. This results in considerable problems in practice. One device designed to solve these problems and which also has succeeded in doing so is described in U.S. Pat. No. 4,519,753. This device has meant a substantial technical progress. This is particularly true as regards a pumping at very high pressures and/or a pumping of slurries of various types. However, this device is comparatively complicated and hence comparatively expensive to produce, partly due to the plurality of pistons, which the system must comprise, and partly because of the high tolerance requirements called for when the pump housing cylinder is manufactured.
As regards plunger pumps some of the difficulties typical of piston pumps can be avoided. However, plunger pumps also have a few problems. This is particularly true as regards plunger pumps performing in strongly contaminated or abrasive media, such as slurries of various types, e.g. cement. Ths basic reason for this is that the plunger during the pump stroke is driven into the pump housing chamber. During the return stroke it is necessary to prevent the pump fluid from coming along past and/or from damaging the sealing devices in the cylinder wall. It has not been possible to solve these difficulties in a satisfactory way in the known devices.
Another problem typical of not only plunger pumps but also other displacement pumps is the pulsating mode of operation. As is the case with the majority of other pump types it is desirable that the pump does not work discontinuously but as evenly as possible. By making two pump units (pump cylinders) work alternatingly in an integrated pump assembly it is possible to reduce the pulsations to a large extent. Regarding mechanically driven displacement pumps there exists certain geometrical relationships between the number of pump cylinders and the degree of non-uniformity, i.e. the variations of the pump flow during a pump cycle. Thus, although it is possible to reduce these variations quite substantially by using a larger amount of cylinders, it is never possible to avoid said geometrical relationships and the corresponding pump flow pulsations. As regards hydraulically driven pumps these inevitable relationships can be broken, since the cylinders can be driven independent of each other, but the problem with pulsations has not been solved in a satisfactory way as regards hydraulically driven displacement pumps according to the state of the art.